Demand for secondary batteries as an energy source has been significantly increased as technology development and demand with respect to mobile devices have increased. Among these secondary batteries, lithium secondary batteries having high energy density, high operating potential, long cycle life, and low self-discharging rate have been commercialized and widely used.
Also, in line with growing concerns about environmental issues recently, a significant amount of research into electric vehicles (EVs) and hybrid electric vehicles (HEVs), which may replace vehicles using fossil fuels, such as gasoline vehicle and diesel vehicle, one of major causes of air pollution, has been conducted. Nickel-metal hydride (Ni-MH) secondary batteries have been mainly used as power sources of the electric vehicles (EVs) and hybrid electric vehicles (HEVs). However, research into the use of lithium secondary batteries having high energy density, high discharge voltage, and output stability has been actively conducted and some of the researches are in a commercialization stage.
A lithium secondary battery is composed of a structure in which an electrode assembly, in which a porous separator is disposed between a positive electrode and a negative electrode in which electrode collectors are coated with each active material, is impregnated with a non-aqueous electrolyte.
In a case in which moisture is included in the lithium secondary battery, it may be a cause of performance degradation of the battery. The moisture in the lithium secondary battery may be included in the active material during a manufacturing process or may be included in the form in which a trace amount is present in an electrolyte solution. For example, a lithium titanium oxide used as a negative electrode active material is a zero-strain material in which structural changes are extremely low during charging and discharging, wherein, since lifetime characteristics are relatively excellent, a relatively high voltage range is available, and dendrites do not occur, the lithium titanium oxide is known as a material having excellent safety and stability. Also, the lithium titanium oxide may have characteristics of fast-charging electrode in which charging is possible within a few minutes, but, in a case in which an electrode is prepared by using the lithium titanium oxide, since the lithium titanium oxide has a property of absorbing moisture in the air, the contained moisture may be decomposed to generate a large amount of gas.
Furthermore, since the moisture present in the electrolyte solution may react with the electrolyte solution due to potential energy provided during the charging, the moisture may reduce reliability of the battery, for example, gas may be generated to cause a swelling phenomenon of a cell. For example, a LiPF6 lithium salt included in the electrolyte solution may react with water to form HF, a strong acid, the formed HF may spontaneously react with the electrode active material having weak basicity to dissolute an electrode active material component, and, as a result, degradation of the battery may occur. Also, since the formed HF may form lithium fluoride (LiF) on the surface of the positive electrode to increase electrical resistance in the electrode and generate gas, the lifetime of the battery may be reduced.
Accordingly, various methods have been used to remove the moisture in the lithium secondary battery. For example, in order to remove moisture in the electrode of the lithium secondary battery, the moisture of the electrode is removed by a high-temperature drying process, and, in order to remove the moisture in the electrolyte solution, a method has been used in which a moisture absorbent is disposed in a battery case (Korean Patent Application Laid-open Publication No. 2015-0037332).
However, since the moisture in the electrode may not be removed to the desired level by only the above method, there is a need to develop a new technique which may improve the battery performance by more effectively removing the moisture in the electrode.